I think the real answer is that users like choices and freedom. You will gain the maximum amount of users if you allow them to use social logins if they so des

I think the real answer is that users like choices and freedom. You will gain the maximum amount of users if you allow them to use social logins if they so desire, or allow them to create a stand alone username/password for your site. By doing it this way you don't lose the users who rebel at social logins, but you still retain the users who prefer social login. The concept of "why not both?" isn't that hard to grasp. I understand that implementing/linking accounts from numerous social logins as well stand alone accounts is more difficult for the developer, but that's the price of maximum growth.